1. Field of the Invention
This invention relates to solid-phase bonding and, more particularly, to the bonding of a first member, such as an external lead, to a second member, such as a thin film terminal, through the utilization of an interposed, specially contoured, compliant medium.
2. Description of the Prior Art
The advent of microminiaturization in the field of electronics and, particularly, the revolutionary changes that have taken place in the design of active and passive solid-state electronic components and circuits, utilizing both integrated and thin film technologies, have presented many serious and peculiar problems with respect to their fabrication and packaging.
The most striking property of integrated and thin film circuits, of course, is their microscopic size. Electrical components prevalent a decade ago, such as transistors and other discrete components, were readily seen and capable of being manually manipulated. Today, such components have been transformed into solid-state circuit elements of almost microscopic size, and often invisible to the unaided eye.
With respect to the packaging of solid-state circuits, the problems encountered are not only related to the minute size of the external leads that must be bonded to the circuits, so as to provide the necessary interconnecting link with associated equipment, but in the number of leads often associated with a given solid-state electronic circuit. The need for a plurality of external leads becomes readily apparent when it is realized that individual integrated circuit chips, often measuring less than a tenth of an inch on a side, may contain many active and passive circuit elements.
Accordingly, while the joining of one or more wires or leads 50 to 100 mils in diameter to mating terminals or pads of comparable size was common with discrete components a decade ago, today the joining of a large number of external leads, often less than ten mils in diameter, or width, to respectively associated thin metallic film terminals or pads is typical in the packaging of solid-state circuits. In such lead packaging, a glass or ceramic substrate is generally employed not only to support the active integrated circuit chips, but to expand the center-to-center spacings of the internal chip leads so as to allow external connections to be made to the outside world. The advent of beam lead sealed junction technology made external lead spacing expansion mandatory, as beam lead center-to-center spacings in the order of 1 to 3 mils has become commonplace.
In manufacture, the external leads may be attached to the branched out thin film paths, terminating in conductive connection areas, either prior to or after the bonding of the beam lead chip(s) to the supporting substrate. While the thin film connection areas are often interchangeably referred to in the art as contacts, lands, pads and terminals, the latter descriptive word will be generally used hereinafter in the interest of consistency.
Primarily due to the micro-size and two-dimensional nature of thin film terminals formed on the supporting substrate, normally by an evaporation process, such terminals are not effective in dissipating energy. Accordingly, during any joining process, not only the mechanical, but thermal energies imparted to the terminals will, in actuality, be transmitted directly to the substrate material. A concomitant problem in making reliable connections to thin film terminals is that the extremely small volume thereof places stringent limitations on the degree of abrasive action, dissolution and/or evaporation that can be tolerated during the metal joining operation.
For the foregoing reasons, it became quite apparent that a very significant and critical factor involved in achieving thin film connection quality, or integrity, is the effect on both the thin film and substrate of the energies imparted thereto by a given joining process. It was such a recognition that led to the conclusion at an early date that conventional bonding (including both soldering and brazing) techniques would generally not prove attractive and, in most cases, even feasible in making lead connections to thin film terminals. Moreover, the use of a low strength joining material, such as soft solder, and an increase in the areas to be joined in order to achieve high overall strength, for example, is inconsistent with the object of achieving a reliable micro-bond or weld zone consistent with high density electronic circuitry. The utilization of conventional solder to bond external leads to thin films, particularly on glass, can also often lead to what is referred to as substrate crazing, caused by solder induced thermal stresses in the substrate.
Fusion welding is another technique attempted heretofore to attach external leads to thin film terminals, but with limited success because of the minute thickness dimensions of the normally evaporated layers forming such terminals. Lasers, electron beams and resistance welders are other heat sources most often used in fusion welding.
In view of the foregoing deficiencies of alloy and fusion bonding (or welding) processes with respect to the assembly and lead packaging of solid-state circuits, in particular, attention was therefore concentrated on other types of joining processes that would not adversely affect the interface physical properties, while at the same time insure reliable lead-terminal adhesion qualities. Such concerted efforts lead to the use of solid-phase bonding which, as the name implies, is normally performed in the absence of any gross melting during bond formation.
The most common forms of solid-phase bonding are categorized broadly as thermocompression and ultrasonic bonding. A solid-phase bond is normally formed by inducing material flow in one or both members to be bonded by the application of heat and/or pressure so that adhesion takes place in the absence of a liquid phase, as in alloy or fusion bonding.
In connection with the external lead packaging of solid-state circuits, solid-phase bonding, particularly when used in conjunction with lead frames, advantageously makes possible low cost, multiple and reliable lead-terminal bonding on an automated basis. The utilization of a lead frame, of course, greatly facilitates the feeding, positioning and aligning of one or more arrays (or sets) of micro-size leads in overlying or underlying relationship relative to the associated thin film terminals on the circuit substrate.
In applications where there are multiple, micro-size lead frame leads and terminals to be solid-phase bonded, either thermocompression or ultrasonic wedge bonding is generally preferred. Such bonding processes normally require very close control over the pressure applied to the mating members, as well as over the heat applied in the case of thermocompression bonding. These parameters are of particular importance when the terminals comprise thin film layers deposited on a glass substrate.
More specifically, as is well known to those active in the manufacture of solid-state circuitry, variations in the flatness of the substrate, whether of glass, ceramic or any other material, may readily vary by as much as 100 microns. Such surface irregularities unfortunately seem to exist regardless of the care taken in polishing or otherwise finishing the bonding surfaces of the substrate. Similar surface irregularities, of course, occur in the evaporated thin films and in the mating surfaces of the leads. As such, these various mating surface imperfections often produce a very serious, cumulative error with respect to parallelism between the multiple bonding wedge tip and the top surfaces of the leads intended to be engaged thereby. This, in turn, can readily produce a considerable variation in spacing between the bonding wedge tip and the aligned leads.
It thus becomes readily apparent that as the number of leads to be bonded is increased, the total force imparted to the common bonding wedge will necessarily increase proportionally. Accordingly, a very detrimental concentration of pressure may often be created at one or more lead-terminal bond sites, leading to actual fracture or cracking of the substrate.
To compensate for and/or to minimize the establishment of such concentrated forces on the substrate, a technique of so-called compliant bonding in one form or another has been employed heretofore. In compliant bonding the substrate is normally mounted on a compliant supporting base which may comprise, for example, a resilient compensating pad, such as of rubber positioned beneath the substrate. A modification of such an arrangement has been the use of a plurality of spaced steel pins interposed between the resilient compensating pad and the underside of the substrate. Various pivotal and sometimes resiliently supported ball type mounts have also been employed heretofore as a compensating and compliant substrate base support.
While such prior compliant bonding techniques have proven to be quite successful with respect to effecting reliable thermocompression or ultrasonic bonds on relatively thick ceramic substrates, such techniques have not always proven to be satisfactory in the bonding of micro-size leads to thin film terminals on many glass and thin ceramic substrates. The reasons for this appear to be multi-faceted.
More specifically, solid-phase thermocompression or ultrasonic bonding is achieved by inducing an appreciable amount of material flow, or plastic deformation, in one or both members to be bonded during the application of pressure (as well as heat in TC bonding), so that effective dispersion of interface contaminants and member-to-member adhesion takes place in the absence of a liquid phase. With respect to plastic deformation, however, the plastic behavior of metals, in particular, is determined not only by external parameters such as temperature and pressure, but by material properties such as crystal structure, purity and microstructure. For these reasons, coupled with the fact that even the most carefully prepared mating bonding surfaces inherently are rough with respect to microscopic standards, it is not surprising that intimate micro-size lead-terminal contact normally takes place at only one or several microscopic bonding sites and, even then many times with deleterious pressure contact, often resulting in substrate fracture.
In addition to the aforementioned factors and problems affecting the integrity of solid-phase bonds, another serious problem encountered heretofore in the bonding of external leads to thin film terminals on glass substrates relates to a phenomenon known as glass cavitation. Such cavitations develop randomly and immediately beneath certain thin film terminals, normally evaporated on the glass substrate, and may best be described as minute, surface glass nuggets which physically separate from the parent substrate material. Such defects appear to result selectively, as well as collectively, from such interrelated factors as insufficient bond compliancy, excessive bonding forces, and insufficient bond definition (interface surface area).
Unfortunately, such glass cavitations almost always result in bonds exhibiting very low or zero pull strength. The serious nature of this condition is compounded by the fact that most of such affected thin film terminals are usually destroyed, making it impractical to repair the almost completely fabricated solid-state circuit. With such circuits often being very complex and quite costly, selective terminal damage due to glass cavitation, as well as substrate fracture due to excessive bonding pressures, constitute particularly costly defects in the manufacture of solid-state devices and circuits.
It is thus seen that conventional thermocompression and ultrasonic bonding of micro-size leads directly to thin film terminals formed on glass substrates has frequently and selectively led to one of the following deleterious conditions: (1) non-uniform bond strengths, (2) substrate fracture, and (3) cavitation of the glass under selective thin film terminals. Extensive investigation led to the conclusion that the three above listed problems are unfortunately interrelated. More specifically, the strength of thermocompression produced bonds could generally be improved by increasing the temperature of the bonding tip, and/or the force applied during bonding, and/or the length of the bonding cycle. Unfortunately, however, varying any of these parameters appreciably from nominal values invariably results selectively, or collectively, in an unsatisfactory degree of bond integrity, glass cavitation and substrate fracture.
Accordingly, there has been an urgent need to modify conventional solid-phase bonding techniques so as to minimize, and hopefully completely eliminate, the aforementioned interrelated bonding problems, particularly with respect to glass substrates. This has led to the utilization heretofore of an interposed contact, such as of gold, between each mating lead and terminal. There have been a number of techniques proposed and/or used heretofore to respectively position such supplemental interposed contacts between mating leads and terminals. One such technique has involved the individual bonding of a micro-size gold contact, e.g., in the form of a disc, on the mating surface of each lead. This has proven to be a very time consuming and costly procedure on a mass production basis and, in addition, because of the contact geometry, does not provide an appreciable amount of resiliency or compliancy without an excessive amount of bonding pressure being employed.
Another technique employed to form an interposed bonding contact heretofore has involved the electrodepositing of a thin gold stripe across the terminating end region of each lead. In the case of lead frames, such a stripe may be formed either before or after the blanking operation. For reasons pointed out in greater detail hereinafter, such a technique has the disadvantage that the electrodeposited stripe cannot be readily and inexpensively formed with the thickness and peculiar cross-sectional configuration necessary to attain consistent, high quality bonds, i.e., bonds exhibiting the requisite solid-phase bond interface properties.
A superficially similar, but non-related technique for forming precious metal contacts on lead frame leads has been employed in the manufacture of electro-mechanical relays. In such an application, the contacts have been attached by resistance (fusion) welding of pre-formed precious metal wires or tapes to relatively thick strip stock, with the leads subsequently being blanked out of the processed strip stock. Such a fusion joining process, however, is normally not applicable for use with strip stock having thicknesses on the order of 1 to 5 mils. Moreover, such wires or tapes do not require any peculiar shaping during welding either for achieving reliable adhesion to the strip stock, as a liquid-phase type of bond is relied upon, or even more importantly, for achieving inherent compliancy, because such pre-formed welded wire or tape segment-contacts are not used as an interposed compliant bonding medium.
Rather, such welded contacts are simply used to insure reliable, non-corrosive, long wearing, periodic relay-actuated contact with a mating member. It thus becomes readily apparent that neither such a lead frame fabrication technique, nor the contacts produced therewith, are of interest with respect to the problems of achieving reliable solid-phase (as distinguished from liquid-phase) bond integrity, while simultaneously eliminating substrate failures in the lead packaging of micro-size solid-state devices or circuits.
Accordingly, in addition to the inadequacies of previous interposed contacts per se, particularly with respect to being shaped to exhibit substantial inherent interface bonding compliancy, such as between a lead and thin film terminal, there has been a need for apparatus to bond and geometrically shape an interposed bonding medium into an inherent compliant contact either during or after attachment, preferably to the leads, in an efficient, reliable, and automated manner. A contact shaping operation, of course, is needed not only to produce an effective compliant contact, but in the case of solid-phase bonded contacts to effect an improvement in both the mechanical cleaning of, and lead-terminal adhesion at both the lead-contact and contact-terminal bond interfaces. This is particularly important when thin film terminals are involved as they are not amenable to directly induced cold flow.
Notwithstanding the need for and the benefits that can be derived from the use of a specially shaped and interposed compliant bonding contact per se, the problem alluded to hereinabove, pertaining to random surface micro-imperfections, particularly in glass substrates, may in very demanding bonding applications, necessitate still additional precautionary measures to be taken in order to insure reliable and constant bond integrity.
More specifically, there has also been a need for the use of not only one initially formed bond, but at least two or more distinct and separate bonds between a lead and terminal, for example, so as to produce what may be referred to as a redundant type of bond therebetween. A redundant bond, as defined herein, consists of forming two or more distinct and separate bonds between two members, with a distinct area between such members being left unbonded, and with a minimum center-to-center spacing being provided between adjacent bonds for reasons described in greater detail hereinafter. Through the utilization of such redundant bonding, investigation has proven that it is statistically improbable that a high quality solid-phase bond will not be produced from at least one of the contact-established bonding sites. Moreover, it is equally statistically improbable, with sufficient compliancy established through multiple interposed contacts, for the aforementioned glass cavitations to develop immediately beneath two or more bonding sites on, or for a fracture to develop in, a glass substrate.
Redundant bonds may also prove to be very important in certain applications because of microscopic substrate surface blemishes caused, for example, by a foreign contaminant or particle that prevents complete thin film adherence to the substrate. When this happens, the defective portion of the terminal may very readily break off from the remainder of the terminal at a single lead-contact-terminal interface. A corollary to this problem, of course, is the possibility that a microscopic surface blemish may similarly occur between a given compliant contact and the lead to which it is mutually bonded. Accordingly, the use of more than one compliant contact per lead to produce redundant bonds can very effectively minimize, if not eliminate, lead-terminal bond failures.
Micro-gap or parallel gap thermocompression bonding, of course, produces a form of redundant bonding. However, such bonding processes do not allow flexible control over the spacings between the electrodes, nor are they conducive to multiple bonding applications utilizing a common heated bonding tool. Parallel gap bonding also does not require, nor has it been utilized, or suggested for use, heretofore, in combination with multiple interposed contacts, whether pre-shaped or not, in order to resolve the aforementioned solid-phase bonding problems and, thereby, achieve the desired end results urgently sought in the art heretofore.